November 11, 2016
The theory of gravity gets a little weird in a few spots in our universe. One of those spots is the outer edges of galaxies. Stars in the outer regions of a galaxy rotate around the galaxy’s center faster than we would expect. This extra speed seems to come from some sort of extra gravitational force, and since we haven’t seen the matter responsible, we’ve basically just stuck dark matter into the theory of gravity to explain the speedy rotation.